In an axial gap electronic motor, as disclosed in, for example, Patent Document 1 (Japanese Patent Application Publication No. 2000-253635), a rotor and a stator are arranged opposedly along the axial direction of an output shaft with a predetermined gap provided therebetween. The axial gap electronic motor can be designed so as to be thin as compared with the ordinary motor. For this reason, the motor of this type is preferably used as a hub motor for a power assisted bicycle, a driving motor for an FD drive, and the like.
In a brushless motor made up of a stator iron core made of a magnetic material and a magnet rotor, when the magnetic resistance between the rotor and the stator is changed by the rotation of rotor, cogging torque is produced by attraction and repulsion in the rotation direction. If this cogging torque is high, vibrations and noise occur.
Especially in the case of the axial gap electronic motor, design is made so that the gap between the rotor and the stator is very narrow. Therefore, there is a fear that the rotor is brought into contact with the stator by vibrations, which may provide a cause of breakage, etc.
Thereupon, as one method for decreasing the cogging torque, a stator iron core structure in which a skew (slope) is provided on a stator iron core has been used. Specifically, the skew can achieve an effect of decreasing cogging torque by tilting a part of stator iron core through a predetermined angle along the direction of rotation of the rotor. On the other hand, however, the skew has a drawback of decreased efficiency of conversion to rotational energy.
In the axial gap electronic motor, the stator iron core structure provided with a skew on the stator iron core has not been used from viewpoints of the connection line of coil on the stator iron core, the assembling ability, and the like. Therefore, as shown in a graph of FIG. 6 showing the relationship between cogging torque and electrical angle, when the energization of each stator iron core is switched over, a high cogging torque is produced on the rotor. It is urgent to solve this problem.